Able to generate deb packages

The following command can generate a Debian package:

    debuild -us uc

The directory structure and tarball must be perfectly set up first. This
is documented in `DEVELOPERS.md`.
Handle relative dotfiles directories

The `lsrc` command (and thus the other commands, too) now supports
passing a relative directory name to `-d`. Previously, this caused only
the non-host and non-tag directories to run; now all dotfiles
directories are run for all metafiles, too.

As an example, this now works correctly:

    lsrc -d dotfiles

This still works, too:

    lsrc -d /home/mike/dotfiles
Alter the dotfiles dir with mkrc

Bugfix: passing `-d` to `mkrc` previously did not make the symlink. This
is now fixed.

We have previously installed the file by calling `rcup`, but we never
passed the `-d` flag to `rcup`. Instead, we `cd`ed. This changes it: no
`cd`, pass the `-d` flag.